The present invention relates to a radio frequency filter the resonator of which is made up of a wire wound into a cylindrical coil comprising a number of turns, of a casing surrounding the coil, and of an insulating plate supporting the cylindrical coil from the inside, and in which the signal lead is connected to the resonator coil via the insulating plate.
Because of their good electrical properties and light weight, filters comprising helix resonators are widely used in radio equipment. The resonator is a transmission line resonator, and it is made up of an approximately quarter-wave wire wound into a cylindrical coil disposed inside a grounded metal casing. The characteristic impedance of the resonator, and thereby its resonance frequency, is determined by the physical dimensions of the cavity, the ratio of the diameter of the helix coil to the inner dimension of the casing, and the distance of the turns of the coil from each other, i.e. the so-called pitch, and the support structure possibly used for supporting the coil. For this reason, preparing a resonator to resonate at precisely a desired frequency requires an accurate and precise structure.
A filter having desired properties can be constructed by series coupling of the resonators and by suitable arrangement of the coupling between them. With decreased filter sizes, especially in portable radio equipment, the precision requirements imposed on their manufacture and assembly increase drastically, since even small dimensional variations in the cavity, the cylindrical coil and the support structure greatly effect the resonance frequency. When a filter is coupled as part of the electrical circuit of a radio device, its input and output ports must be matched with the circuit, i.e. the impedances exhibited from the ports towards the filter are made the same as the impedances exhibited from the ports towards the circuit, so that reflections caused by a sudden change of impedance do not occur in the ports thereby reducing transmission losses. Likewise, the filter resonators must be matched with each other if a signal is introduced into the filter by physical coupling to its helix coil.
Thus a suitable impedance level must be found in the resonator, i.e. a physical connection point at which the impedance level from the connection point towards the resonator corresponds to the impedance level of the device to be coupled to it or of the adjacent resonator.
The impedance level of the connection point is directly proportional to the distance of the connection point from the short-circuited end of the resonator, in which case a higher or lower impedance level can be selected by changing the connection point on the helix coil. This matching is called tapping, since the connection point forms a tap from the helix resonator. The tap point may be determined experimentally or be calculated by using the resonator's calculated or measured characteristic impedance, which in turn is proportional to the electrical length of the resonator. Often the tap point in a helix resonator is on its first turn.
Tapping has conventionally been done by soldering, at the tap point, one end of a separate coil or wire to the wire forming the helix resonator. With decreasing filter sizes, the reproduction fidelity of such a tapping method is inadequate for mass production. Inadequate tapping accuracy results to a need for adjusting the taps in the process of tuning the filters; this slows down the tuning and increases costs.
An improved tapping method is described in Finnish patent 80542. The principle is shown in accompanying FIG. 1. The helix resonator 6 is disposed on a finger-like projection 3 of an insulating plate I in such a manner that the projection is within the resonator coil and supports the coil. At that end of the coil 6 which is towards the insulating plate 1, the beginning of the first turn is bent into a straight portion 2, the entire length of which is tightly against the surface of the insulating plate. The straight portion is called in the art the resonator leg. The end 7 of the portion 2 is in contact with the casing 5, being thereby short-circuited. The insulating plate has, at the base of the projection 3, a microstrip line 8 which is in contact with the rest of the resonator circuit or is part of a more extensive microstrip line pattern on the insulating plate. The microstrip line is parallel to the coil axis. The tap point is in this case the point at which the microstrip line 8 intersects with the straight portion 2 of the coil. The stripline and the straight portion are soldered to each other at this point. The tap point, and thereby the desired impedance level, is determined by moving the position of the microstrip line 8 in the lateral direction.
This method has the disadvantage that the changing of the impedance level of the tap point requires a large number of insulating plates different with respect to the lateral position of the microstrip line. This is a cost-increasing factor. Another disadvantage is that fine adjustment of the tap point is impossible, since the leg must come against the insulating plate. In practice the leg being against the insulating plate is not a very good solution, since the leg against a dissipative plate will increase resonator dissipation.
There is a well known prior art filter in which tapping is done on a stripline in contact with the edge of the finger-like projection described above. Such a filter is depicted in FIGS. 2, 3 and 4, in which the same reference numerals are used for applicable parts as in FIG. 1. FIG. 2 shows a part within the casing of a four-circuit filter, the part comprising four discrete helix resonators--separate reference is made to resonators 6 and 7--each of which is disposed around a finger-like projection 3 of the circuit board 1. In this case, the term used in the art is `comb structure`. In the lower section 1A of the insulating plate 1 there is an electric circuit formed of striplines 8, 8', to which one or more resonators, such as resonator 6, is coupled at the tap point 21 by soldering. The tap point is here at the first turn of the coil, but it may just as well be higher. This possibility is illustrated by resonator 7 in FIG. 2, in which the tap point 22 is at the second turn of the coil. In this case the stripline extends somewhat upwards in the finger-like projection and ends at the projection edge, at which the soldering takes place to the resonator turn which is at that point. The tap point may thus be at any resonator turn, and there may be a number of tap points. The straight leg 2 of the resonator has, in a manner different from the leg in FIG. 1, been bent to be parallel to the resonator axis, running at a distance from the insulating plate, and at the assembling stage its other end connects to the bottom plate 31, FIG. 3, of the casing, and is thereby grounded if the plate is of metal. The bottom plate of the casing may also be made up of a circuit board of the radio device, at least one surface of the circuit board in the filter area being metallized throughout, in which case the tip of the leg is connected to the metallized surface.
FIG. 4 depicts a completed filter according to the state of the art, the filter casing 41 being shown partly as a cutaway so that the resonator is clearly visible. This filter has, between the circuits, partition walls, of which walls 42 and 43 are visible, which may have a coupling aperture (not shown in the figure) through which the circuit can be coupled by an electromagnetic field to the adjacent circuit. The partition wall has no significance in terms of the invention, nor does the manner in which the insulating plate supporting the resonators is attached to the casing walls. The casing 41 is most commonly an aluminum casing manufactured by extrusion, and the bottom plate 44 may be a metal plate or a circuit board one surface of which is metallized. The tap points 21 and 22 of the helix resonators 6 and 7 which are visible are indicated with black dots, and at this tap point the resonator connects to a stripline circuit (not shown in the figure) made in the lower section 1A anti fingers 3 of the insulating plate. The tips 12 and 13 of the legs 2 and 2' are soldered to the bottom plate 44 if the plate or its surface is of metal, or they are electrically connected to a metal foil on the opposite side of the bottom plate if the bottom plate is a circuit board.
The structure depicted in FIGS. 2 and 3 has certain disadvantages. In order for the tapping, and thereby the impedance exhibited at the tap point, to be precisely correct, the helix coil must be placed in precisely the correct position on the finger-like projection 3, so that the distance, measured along the coil, from the tap point to the grounded tip of the leg will be precisely correct. Even the slightest rotation relative to the axis of the coil will change the tap point and thereby the impedance. In the manufacturing of a filter, the position of the helix coil, when it is placed automatically on the projection, will vary owing to process variation, whereupon the electrical and physical height of the tap point from the ground potential will vary. In manufacture this will cause variation in the properties of filters. Control of the variation is very difficult, especially when the operation takes place at the limits of precision of the production process. So far, the only solution to this problem has been to make efforts to carefully control the precision of the process.